Electrion discharge device with hollow resonator



Sept. 26, 1950 A. F. PEARCE EIAL ,776

ELECTRON DISCHARGE DEVICE WITH HOLLOW RESONATOR Filed Jan. 16, 1946 ALBERT FREDERICK PEARCE NORMAN CHARLES BARFORD BERNARD JOSEPH MAYO BY ATTORNEY Patentecl Sept. 26, 1950 ELECTRON DISCHARGE DEVICE WI'ilH HOLLOW RESONATOR Albert Frederick Pearce, Hampton Hill, and Norman Charles Barford and Bernard Joseph Mayo, Hayes, England, assignors to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application January 16, 1946 Serial No. 641,590

'In Great Britain December 16, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires December 16, 1961 6 Claims.

This invention relates to electron discharge devices with hollow resonators and has exclusive reference to the type of device comprising an electron beam source, a resonator and a reflecting electrode system adapted to reflect the electron beam, after the latter has passed through the resonator, back into the resonator so that the device can function as a generator of oscillations.

In such devices the hollow resonator, which may be of suitable toroidal form, or of other suitable forms as have heretofore been proposed, is usually maintained at about 1000 to 2000 volts positive with respect to the cathode. The electrons are reflected due to the field set up by the reflecting electrode system which usually comprises a single reflecting electrode maintained at a negative potential of some hundred volts or more with respect to the cathode. The beam is reflected back through the apertures in the resonator without impinging on the reflecting electrode and in the space between the resonator and the reflecting electrode the velocity modulation which is imparted to the beam on passing through the resonator, becomes converted into charge density modulation. In this type of device there are, in general, two conditions to be satisfied. Firstly, the time taken for the electrons to pass from the centre of the gap between the apertures in the resonator to the reflecting region and back to the resonator must be such that the charge density modulated beam returns through the resonator in the correct phase. This transit time is usually arranged to be equal to multiplied by the periodic time of the oscillations where n is any integer which in practice is generally less than six. Secondly, the electric field in the reflecting region must be soshaped that the majority of the electrons are, in fact, reflected back through the resonator, otherwise inefliciency results. It will be appreciated therefore that the shapes and spacings of the electrodes bounding the reflecting region determine largely the efficiency of the device in operation and that the potential applied to the resonator and the reflecting electrode for a given efiiciency are mutually dependent. Thus, for a predetermined potential applied to the resonator there is a series of discrete potentials to be applied to the reflecting electrode, and vice versa,

which afford a series of optimum efiiciencies.

The efliciency in these circumstances therefore depends critically upon the actual potentials applied to the resonator and the reflecting electrode, with. the result that if a particular resonator potential is selected and the reflecting electrode potential is adjusted to afford the optimum eiflciency for such resonator potential then any slight changes from such potentials will cause a decrease intthe optimum efliciency. It is generally found that if one of these potentials is increased the optimum value of the other potential decreases, i. e., the potential should be changed in opposite senses. This is advantageous since it will be appreciated that with the supply arrangements usually employed a change of supply potential occasioned, for example, as a result of a fortuitous fluctuation of the mains supply which aflects the potentials applied to the reso-' nator and the reflecting electrode in the same sense produces a reduction in the optimum ei-.

'one which will enable the frequency of the generated oscillations to be changed over a wide range without causing a substantial reduction in the available output.

It will be appreciated that when operating potentials are applied to the resonator and the reflecting electrode system a potential fleld is set up between the resonator and the said system. Now, we have found that, by making the potential gradient small at the zero equi-potential surface which is set up between the resonator and the reflecting electrode system, not only can the operating potentials be varied in the same sense over a wider range than has heretofore been possible without causing a large reduction in optimum efllciency, but that the frequency of the generated oscillations can be changed by potential variation over a wide range without causing a substantial reduction in the available output.

According to one feature of the invention there is provided an electron discharge device of the type referred wherein the reflecting electrode system is such that the potential gradient set up between the resonator and the reflecting electrode system when appropriate operating potentials are applied to the resonator and the reflecting electrode system is small at the zero equipotential surface.

It will of course be appreciated that where a device according to the invention is used as a generator of oscillations the device can be used in a circuit without expensive voltage stabilising devices, since the potentials applied to the resonator and the reflecting electrode system can both vary in the same sense within a larger range than heretofore without causing a substantial change in the efficiency of the device. In such a case it will be understood that any fortuitous change of the supply potentials which might occur in practice would affect both voltages simultaneously and would not therefore cause the frequency of the generated oscillations to be substantially changed. Where, however, a device according to the invention is employed in an automatic frequency control circuit then the potential applied either to the resonator or to the reflecting electrode system must be changed over a fairly wide range in order to force the oscillator to change its frequency of oscillation.

The required potential gradient can be obtained mainly by employing an elongated reflecting electrode system of suitable shape. Various examples of electrode systems suitable for use in the invention will be described more fully hereinafter.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings in WhiCh- Figure l is a curve illustrating the potential distribution between a resonator and a reflecting electrode system in a device according to the invention,

Figure 2 illustrates a series of curves indicating the relationship between the potentials applied to the resonator and to the reflecting electrode,

Figure 3 is a diagrammatic view of a known form of electron discharge device of the type referred to, 7

Figures 4, 5 and 6 illustrate electron discharge devices of the type referred to constructed in accordance with the invention, and

Figure '7 is a diagram of a superheterodyne receiver embodying an automatic frequency control circuit.

As stated above, the two important results of the present invention arise by making the potential gradient small at the zero equi-potential surface. It will be understood that the term zero equi-potential surface means an imaginary equi-potential surface in space which has a potential corresponding to the potential of the cathode. Thus, if the cathode is maintained, as is possible, at a negative potential of 1000 to 2000 volts and the resonator at zero potential, then the zero equi-potential surface will of course be that surface which has the potential of the cathode.

Figure 1 of the accompanying drawings illustrates the potential distribution set up along the axis of the device between the zero equi-potential surface and the centre of the gap in the resonator, the potential being plotted as ordinates and the distance between the centre of the gap in the resonator and the zero equi-potential surface as abscissae. The centre of the gap is the point lying midway between those surfaces of the reso nator which define the gap. As stated above, the present invention resides in making the potential gradient at the zero equi-potential surface small. By the term small when used herein in connection with a potential gradient We mean a potential gradient which is 0.6 or less of the gradient averaged over the distance between the centre of the resonator gap and the zero equi-potential surface. Thus, no matter what the magnitude of the averaged gradient is between the centre of the resonator gap and the zero equi-potential surface, the important results of the present invention arise by making the gradient at the zero equi-potential surface 0.6 or less of the averaged gradient. It is preferable in a device which is required to be less susceptible to changes in supply potentials and necessary in a device employed for automatic frequency control that the difference between the potential of the zero equi-potential surface and the potential averaged between the zero equi-potential surface and the centre of the resonator gap is not less than one-third of the total difference of potential between the zero equi-potential surface and the centre of the resonator gap. Preferably, however, the gradient is 0.4 to 0.3 of said averaged gradient and preferably said difference is larger than one-third.

Referring to Figure 2 of the accompanying drawings, curve a indicates the relation between a series of negative potentials applied to the reflecting electrode and a series of positive potentials applied to the resonator for affording optimum efficiency at a given frequency. The curve a is typical of the known form of device shown in Figure 3 of the drawings in which the reference numeral 6 indicates a hollow resonator of toroidal form having the cross-section shown. The cathode of the device is indicated by the ref erence numeral 1 and the reflecting electrode system comprises a single reflecting electrode of shallow dish-form indicated by the reference numeral 8. The envelope I of the device is shown in Figure land is omitted in Figures 3, 5 and 6 for the sake of clarity. The surface of the resonator 6- adjacent the reflecting electrode 8 in Fig. 3 is of the curved re-entrant form shown. It will be seen from curve a that if the potential applied to the resonator is increased then the potential applied to the reflecting electrode should be decreased and vice versa for optimum efliciency with the changed resonator potential, 1. e., the potentials should be changed in opposite senses. This is disadvantageous since it will be appreciated that with the supply arrangements usually employed, should the supply potentials change, then a change in efficiency will result owing to the fact that when the positive potential applied to the resonator increases the negative potential applied to the reflecting, electrode also increases, whereas in order to maintain the optimum efiiciency the negative potential applied to the reflecting electrode should be decreased. The potential gradient at the zero equi-potentiaI surface in a device such as shown in Figure 3 would be substantially larger than 0.6 of the averaged gradient and it is found that by making the gradient less than 0.6 the operating potentials of the device can be changed over a wider range in the same sense without causing such a large change in optimum efficiency.

In accordance with our invention, in order to make the gradient small the reflecting electrode 8 of the device as shown in Figure 4 is in the form of an elongated hollow cylinder, that is to say, its axial length exceeds its diameter. It is considerably longer than the reflected electrode shown in Figure 3 and surrounds the beam for a considerable length. The rear surface of the resonator G is preferably made substantially fiat in the vicinity of the electrode 8. The curve I) of Figure 2 of the drawings indicates the relation ship between the resonator potential and the potential applied to the reflecting electrode of the device shown in Figure 4. It will be observed that this curve has substantially no slope and hence if the supply potentials should change in the same sense during operation the reduction in efliciency which would result will not be so large as the reduction which would result when employing a device as shown in Figure 3.

Although the device in Figure 4 provides some improvement compared with the'device in Figure 3, it is possible to obtain a further improve ment by employing an elongated reflecting electrode 8 of the form indicated in Figure 5 of the drawings, this electrode comprising a hollow frustum of an inverted cone, i. e., its apex is directed towards the rear surface of the resonator 6. In a device designed to generate oscillations of 10,000 megacycles per second the diameter of the aperture at the apex of the reflecting electrode may be 3 the angle at the apex 110, and the length of the electrode along the axis of the beam may be 6 mms. The diameter of the aperture in the surface of the resonator adjacent the reflecting electrode may be 1 mm. and'the distance between said surface and the reflecting electrode may be 0.1 mm. The result of employing the construction of electrode shown in Figure 5 is that an increase in potential applied to the resonator also requires an increase in potential applied to the reflecting electrode or viceversa for optimum efiicienoy, i. e. the potentials are required to change in the same sense. The relationship between the resonator potential and the potential applied to the reflecting electrode in a device employing the construction shownin Figure 5 is of the form indicated by curve 9 in Figure 2. From this curve it will be appreciated that as the potential applied to the resonator is increased the potential applied to the reflecting electrode must also be increased for substantially the same degree of efficiency. Consequently, it will be realised that, in operation should all the supply potentials change in the same sense the efliciency of the device will not substantially change. a

The constructions shown in Figures 4 and 5 will n therefore, for a .given set of operating potentials adjusted to afford optimum efficiency, maintain a more constant output whilst allowing the opcrating potentials to vary over a wider range than will the construction shown in Figure 3 and are therefore easier to operate in practice and do not require the use of expensive voltage stabilisers;

The constructions shown in Figures 4 and 5 do "ferred to above.

In Figures 4 and 5 or the drawings, the flat rear surfaces of the reflecting electrodes 3 lay no part in determining the-field conditions and in fact the reflecting electrodes 8 of these two.

figures can be regarded respectively as an infinite tube and as an infinite cone; consequently, the flat rear surface can be omitted if desired.

'By making the potential gradient small at the Zero equi-potential surface it is possible to cause the frequency of the oscillations generated by the device to be changed by variation of the potential applied-to the resonator or the reflecting electrode over a wide range without causing a substantial reduction inthe available output, thus enabling the device to be used with advantage in automatic frequency control circuits. Such circuit is shown schematically in Figure 7 of the drawings. The circuit illustrated is of thesuperheterodyne ,type and received signals from an aerial 9 are fed to a mixing stage indicated at it where the received signals are mixed with local oscillations generated by an oscillator H, the oscillator comprising a device according to the invention. The intermediate frequency output from the mixer is amplified in an intermediate frequency amplifier 12 the output from the amplifier l2 being fed to a detector l3 and to a known form of discriminator circuit M which provides an output depending on the departure of the intermediate frequency from its assigned value. The output from the discriminator circult I4 is fed to the local oscillator H and serves to control the frequency of the oscillations generated by said oscillator in such a manner that the intermediate frequency signals are mainr tained at the correct frequency. The output from the detector i3 is fed to, a low frequency amplifier i5. 7

When employing the device shown in Figure 4 for automatic frequency control purposes the. ratio of the diameter of the aperturev t in the flat rear surface of the resonator to the inner diameter of the tubular electrode 8 may vary from 111.5 to 1:4, For example, the diameter of the aperture may be 0.040" whilst the tubular electrode may have a length of 0.5 and a diameter of 0.160 and may be spaced from said surface of the resonator by a distance of 0.004". In

operation, the resonator may be maintained at a positive potential of 1600 volts with respect to the cathode of the device and the reflecting electrode at a negative potential of 180 volts. Such a deviceprovides a frequency change of 20-30 megacycles per second without a substantial reduction in output. The term substantial reduction means a reduction'exceeding half the power available when the resonator and thereflecting electrode are at their optimum-potentials.

In a further form of device employing tubular reflecting electrode said ratio may be 1 :3, i. e., the diameter of the tubular electrode may be 0.120", the otherdimensions andthe operating potentials being the same as the example above referred to. In this case a. frequency change of 40 megacycles per second was obtained without a substantial reduction in output.

Instead of employing a single tubular electrode as the reflecting electrode system, the latter may comprise, "in a further form of the invention, illustrated in Figure 6, a reflecting electrode proper l6, which may be of concave form, and a tubular electrode i1 interposed between the resonator and the reflecting electrode Iii, the tubular electrode in this case being maintained at a potential intermediate the potential of the resonator and the reflecting electrode it.

In devices according to the invention, it is preferred that the distance between the centre of the gap in the resonator and the zero equi-potential surface should be large so that the zero equipotential surface is well removed from the gap. For example, in the devices described above, for operating with a mean frequency of about 10,000 megacycles per second, the distance between the centre of the gap and the zero equi-potential surface may be 2.6 mms. Where devices according to the invention are employed in automatic frequency control circuits it is desirable that the potential applied to the reflecting electrode should be fairly low, a typical example being that stated above when the potential applied to the resonator is 1600 volts. Where, however, devices according to the invention are not required for automatic frequency control purposes, the voltage'applied to the reflecting electrode can be substantially higher.

The construction of devices referred to above are suitable where the electron beam is of circular form in cross-section, but it will be appreciated that the invention is also applicable to devices in which the beam is of non-circular form. For example, where the beam is of ribbon-shape, the reflecting electrode system must be suitably shaped to deal with the ribbon-shaped beam. For example, instead of employing a tubular electrode of circular form in cross-section,

the reflecting electrode may be of elongated form in cross-section or may simply comprise a pair of fiat parallel plates. The invention can also be applied to devices to which the beam is of annular form and passes through an annular gap ing the sections shown in Figure 4, or 6 about i an axis spaced from and parallel to the axis of the resonator shown in these figures. The crosssectional shape of reflecting electrode systems in devices where the beams are of non-circular form in cross-section may be similar to the shape of the aperture in the near surface of the resonator and may have a similar ratio of dimensions as those described above. It will be understood that the potential gradient referred to is the gradient along the axis of the electron beam where the latter is of circular form in cross-section since in this case the reflecting field is circularly symmetrical and in cases where the beam is of different form in cross-section the potential gradient is the gradient along a line which corresponds to the axis of the beam in the case where the latter is of circular form.

Whilst a device can be constructed according to the invention which is especially suitable for use in automatic frequency control circuits, it will be appreciated that such a device is also suitable for other purposes. For example, the device can be used as the modulator in a frequency-modulated transmitting system.

It will be appreciated that the actual construction of reflecting electrode system is relatively immaterial and'that other forms of reflecting systems might be employed whilst still maintaining the potential gradient small at the zero equipotential surface.

What we claim is:

1. An electron discharge device having a single apertured cavity resonator, cathode means for directing a stream of electrons through said resonator, and a reflecting electrode structure ineluding an elongated hollow conducting electrode comprising a tubular member having a longitudinal axis disposed axially of the path of said electron stream and having an open end disposed adjacent the rear wall of said resonator to receive said electron stream whereby when appropriate operating potentials are applied to the resonator and the reflecting electrode structure the potential gradient set up between the resonator and reflecting electrode structure is small at the zero equipotential surface.

2. An electron discharge device in accordance with claim 1, wherein the cross-sectional shape of said hollow electrode is similar to the shape of the aperture in the rear surface of the resonator and wherein the ratio of the dimensions of the aperture in the resonator to the smallest cross-sectional dimensions of said electrode is 1 1.5 to 1 :4.

3. An electron discharge device having a single apertured cavity resonator, cathode means for directing a stream of electrons through said resonator, and a reflecting electrode structure including an elongated hollow conducting electrode comprising a hollow frustrum of an inverted cone disposed axially of the path of the electron stream and having an open end disposed adjacent the rear wall of said resonator to receive said electron stream, whereby when appropriate operating potentials are applied to the resonator and the reflecting electrode structure, the potential gradient set up between the resonator and the reflecting electrode structure is small at the zero equipotential surface.

4. An electron discharge device having a single apertured cavity resonator, cathode means for directing a stream of electrons through said resonator, and a reflecting electrode structure including an elongated hollow conducting electrode comprising an elongated hollow cylinder disposed axially of the path of said electron stream and having an open end disposed adjacent the rear wall of said resonator to receive said electron stream, whereby when appropriate operating potentials are applied to the resonator and the reflecting electrode structure, the potential gradient set up between the resonator and the reflecting electrode structure is small at the zero equipotential surface.

5. An electron discharge device in accordance with claim 1, wherein said reflecting electrode structure further includes a relatively fiat electrode disposed normal to the path of said electron;

stream and spaced from said resonator by said tubular member.

ALBERT FREDERICK PEARCE. NORMAN CHARLES BARFORD. BERNARD JOSEPH IWAYO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,120,974 Foster June 21, 1938 2,128,232 Dallenbach Aug. 30, 1938 2,170,219 Seiler Aug. 22, 1939 2,220,840 Metcalf Nov. 5, 1940 (Other references on followin pag Number r 9 UNITED STATES PATENTS Name Date Varian et a1 July 29, 1941 Hansen et a1 Oct. 21, 1941 Varian et a1 June 30,1942 5 Linder Mar. 23, 1943 Litton Mar. 27, 1945 Number Number 10 Name 1 Date Ginzton et a1 Dec. 18, 1945 Pierce Sept. 3, 1946 Hill Mar. 18, 1947 FOREIGN PATENTS Country Date Germany Sept. 29, 1938 

